The studies described in this research project are aimed at defining the biochemical changes that affect platelets' primary functions. Biological changes occur when platelets are withdrawn from their normal hemostatic environment and held stored, that is, before they are used either for studying in vitro their functions or for transfusion into patients. Platelets in vivo are sturdy fragments of larger cells that undergo irreversible changes when stimulated. In vitro, the half-life of platelets is extremely short when compared with other blood cells. Their normal activities become impaired because it is not known which parameters undergo biochemical decay. As a result of our recent investigations to understand the basic processes in platelets, we have investigated several ultrastructural constituents of platelets that suffer early deterioration. Also we have tested several reagents and conditions capable of arresting this change. As a result of our efforts to understand the basic processes in platelets we have advanced or knowledge of how to prevent rather than delay their deterioration; indeed, we have maintained platelet stability with improved biological activity. We believe that this increased understanding will lead to a method for improved platelet-storage conditions that should permit prolonged survival in vivo and optimal preservation in vitro . We are investigating the elements that maintain cytoplasmic proteins organized in a dynamic equilibrium between their monomeric and polymeric states. When platelets are stimulated, their cytoplasms become either jellified or liquified by distinct processes of polymerization and depolymerization at precise sites. This produces either self-adherence of various platelets, partial or total release of granules, and ultimately formation of a thrombus.